Electromagnetic surveying for hydrocarbon reservoirs

ABSTRACT

A method of electromagnetic surveying of an area of seafloor that is thought or known to contain a subterranean hydrocarbon reservoir is described. The method includes broadcasting an EM signal from a horizontal electric dipole (HED) transmitter and obtaining vertical electric dipole (VED) response data at a remote receiver in response thereto. Survey data are analysed by comparing the VED response data with background data which are not sensitive to the postulated hydrocarbon reservoir. Accordingly, differences between the VED response data and the background data allow for the identification of buried hydrocarbon reservoirs. The background data may be provided by magneto-telluric surveying, controlled source electromagnetic surveying or from direct geophysical measurement. By employing VED response data in this way, surveys may be performed in shallower water than has previously been possible since the VED detector is not sensitive to air-wave components of the EM field induced by the HED transmitter at the VED detector.

This application is a national phase of International Application No.PCT/GB2004/001877 filed Apr. 30, 2004 and published in the Englishlanguage.

BACKGROUND OF THE INVENTION

The invention relates to seafloor electromagnetic surveying for oil andother hydrocarbon reserves.

Determining the response of subterranean strata within the earth's crustto electromagnetic fields is a valuable tool in the field of geophysicalresearch. The geological structures associated with thermally,hydrothermally, tectonically or magmatically active regions can bestudied. In addition, electromagnetic surveying, or sounding, techniquescan provide valuable insights into the nature, and particularly thelikely hydrocarbon content, of subterranean reservoirs in the context ofsubterranean oil exploration and surveying.

Seismic techniques are often used during oil exploration to identify theexistence, location and extent of reservoirs in subterranean rockstrata. Whilst seismic surveying is able to identify such structures,the technique is often unable to distinguish between the differentpossible compositions of pore fluids within them, especially for porefluids which have similar mechanical properties. In the field of oilexploration, it is necessary to determine whether a previouslyidentified reservoir contains hydrocarbons or just aqueous pore fluids.To do this, an exploratory well is drilled to determine the contents ofthe reservoir. However, this is an expensive process, and one whichprovides no guarantee of reward.

Whilst hydrocarbon-filled and water-filled reservoirs are mechanicallysimilar, they do possess significantly different electrical propertiesand these provide for the possibility of electromagnetic baseddiscrimination testing.

A known technique for electromagnetic probing of subterranean rockstrata is the passive magneto-telluric (MT) method. Signals measured bya surface-based electromagnetic receiver in response to electromagnetic(EM) fields generated naturally, such as within the earth's upperatmosphere, can provide details about the surrounding subterranean rockstrata. However, MT fields excite predominantly horizontal current flowsin the earth and this makes MT surveying intrinsically insensitive tothe thin resistive layers typical of subterranean hydrocarbonreservoirs. Furthermore, MT data are rarely collected at the seafloor atfrequencies high enough to resolve subterranean strata on scales typicalof hydrocarbon reservoirs. In addition, the effect of distant coastlinescan also often be seen in MT data. This increases the complexity of datainterpretation. Notwithstanding these limitations, MT techniques arestill useful for determining large-scale background structure in asubterranean strata configuration, even if they cannot be directlyapplied to surveying for subterranean hydrocarbon reservoirs [7].

Because of the different electrical properties of hydrocarbon-filled andwater-filled reservoirs, measurements of electrical resistivity beneaththe seafloor have traditionally played a crucial role in hydrocarbonexploration and reservoir assessment and development. In industry,subterranean resistivity data have generally been obtained almostexclusively by wire-line logging of wells. There are, though, clearadvantages to developing non-invasive geophysical methods capable ofproviding such information from the surface or seafloor. Although suchmethods are unlikely to provide comparable vertical resolution towireline logging, the vast saving in terms of avoiding the costs ofdrilling test wells into structures that do not contain economicallyrecoverable amounts of hydrocarbon would represent a major economicadvantage.

In research fields that are not of commercial interest, geophysicalmethods for mapping subterranean resistivity variations by various formsof controlled source EM surveying have been in use for many years [1, 2,3, 10]. Proposals for finding hydrocarbon reservoirs using such EMsurveying techniques have also been made [4, 5], and applications to thedirect detection of hydrocarbons using horizontal electric dipole (HED)transmitters (or sources) and receivers (or detectors) have provedsuccessful [6, 7].

FIG. 1 schematically shows a surface vessel 14 undertaking controlledsource EM surveying of a subterranean strata configuration according toa previously proposed method [6]. The subterranean strata configurationincludes an overburden layer 8, an underburden layer 9 and a hydrocarbonreservoir 12. The surface vessel 14 floats on the surface 2 of a body ofwater, in this case seawater 4 of depth d meters. A submersible vehicle19 carrying a HED transmitter 22 is attached to the surface vessel 14 byan umbilical cable 16 providing an electrical and mechanical connectionbetween the submersible vehicle 19 and the surface vessel 14. The HEDtransmitter is supplied with a drive current so that it broadcasts a HEDEM signal into the seawater 4.

One or more remote receivers 25 are located on the seafloor 6. Thereceivers 25 include an instrument package 26, an antenna 24, afloatation device 28 and a ballast weight (not shown). The antenna 24comprises an orthogonal pair of horizontal electric dipole detectors.The horizontal electric dipole detectors are sensitive to EM fieldsinduced by the HED transmitter in the vicinity of the receiver 25, andproduce detector signals therefrom. The instrument package 26 recordsthe detector signals for later analysis.

The HED transmitter 22 broadcasts EM signals that propagate outwardsboth into the overlying water column 4 and downwards into the seafloor 6and the underlying strata 8, 9, 12. At practical frequencies for thismethod and given the typical resistivity of the respective media 4, 8,9, 12, propagation occurs by diffusion of electromagnetic fields. Therate of decay in amplitude and the phase shift of the signal arecontrolled both by geometric spreading and by skin depth effects.Because in general the underlying strata 8, 9, 12 are more resistivethan the seawater 4, skin depths in the underlying strata 8, 9, 12 arelonger. As a result, electric fields measured by a receiver located at asuitable horizontal separation are dominated by those components of thetransmitter EM signal which have propagated downwards through theseafloor 6, along within the underlying strata 8, 9, 12, and back up tothe receiving antenna 24. Both the amplitude and the phase of thereceived signal depend on the resistivity structure of the underlyingstrata 8, 9, 12. Accordingly, a survey built up from many transmitterand receiver locations can provide a multi-dimensional image, bygeophysical inversion, of subterranean resistivity. Because hydrocarbonreservoirs have relatively high resistivities (typically 100 Ωm)compared to other subterranean strata (typically 1 Ωm), they can beeasily identified in maps of subterranean resistivity.

However, a significant problem with controlled source EM surveyingtechniques of the kind shown in FIG. 1 is that they do not work well inshallow water due to the presence of an ‘air-wave’ component in the EMfields induced by the HED transmitter at the receiver. This air-wavecomponent is due to EM signals from the HED transmitter which follow apropagation path upwards through the seawater to the surface;horizontally through the air; and back down through the water column tothe receiver. The air-wave component contains very little informationabout subterranean resistivity. Accordingly, if the air-wave contributesa significant component to the EM fields induced by the HED transmitterat the receiver, the sensitivity of the technique to subterraneanresistivity structures, such as hydrocarbon reservoirs, is greatlyreduced. The path of an example air-wave component is shown in FIG. 1 bya dotted line labelled AW. The magnitude of the air-wave component isnot significantly reduced by its passage through air. This is becauseair is non-conducting. However, as with other components, the airwavecomponent is strongly attenuated by its passage through the seawater.This means that in relatively deep water (large d) the air-wavecomponent is not very significant at the receiver and as such does notpresent a major problem. However in shallow water (small d) the airwavecomponent does not pass through as much seawater and thus makes a largercontribution to the EM fields induced by the HED transmitter at thereceiver. This contribution becomes greater still at increasingtransmitter-receiver horizontal separations. This is because (other thandue to geometric spreading) the strength of the air-wave component isrelatively constant over a wide range of horizontal separations sinceany extra distance travelled by the air-wave component is almostexclusively in the non-attenuating air. Other components of the EMfields induced by the HED at the receiver, such as those which passthrough the subterranean strata and are of interest, travel throughlower resistivity media and become increasing attenuated as they travelfurther. For these reasons, the air-wave component tends to dominate theEM fields induced by the HED transmitter at the receiver for surveysmade in shallow water, especially at long transmitter-receiverhorizontal separations.

The existence of the air-wave as a dominant component of the detectorsignals limits the applicability of the above described surveyingtechnique. In shallow water the range of transmitter-receiver over whichthe technique can be applied is much reduced. This not only leads to aneed to employ more receiver locations to adequately cover a given area,but also limits the depth beneath the seafloor to which the technique issensitive. This can mean that a buried hydrocarbon reservoir in shallowwater may not be detectable, even though the same reservoir would bedetected in deeper water.

FIG. 2 is a graph schematically showing results of one-dimensionalmodelling of two example EM surveys of the kind shown in FIG. 1. Oneexample corresponds to a survey performed in deep water (dotted line)and the other to a survey performed in shallow water (solid line). Foreach model survey the amplitude of an electric field component inducedat the receiver in response to the HED EM transmitter is calculated perunit transmitter dipole and is plotted as a function of horizontalseparation R between the HED transmitter and the receiver. For bothmodel surveys, the subterranean strata configuration is a semi-infinitehomogeneous half space of resistivity 1 μm. In the deep-water example,the subterranean strata configuration is located beneath an infiniteextent of seawater. In the shallow-water example, it is located beneatha 500-meter depth of seawater. In both cases the seawater hasresistivity 0.3 μm. The transmitter and receiver are separated along aline which runs through the axis of the HED transmitter. It is thecomponent of detected electric field resolved along this direction whichis plotted in FIG. 2. The HED transmitter is driven by a quasi-squarewave AC current at a frequency of 0.25 Hz.

The effect of the air-wave component on the amplitude of EM fieldsinduced by the HED transmitter at the receiver is clear. In thedeep-water model survey, where there is no air-wave component, thecalculated electric field amplitude falls steadily with increasinghorizontal separation. In the shallow-water model, however, where thereis a strong air-wave component, the rate of amplitude reduction sharplyreduces at a transmitter-receiver horizontal separation of about 5000 m.FIG. 3 is a plot showing the ratio, p, of the two curves shown in FIG.2, and the large deviations from unity highlight the difference betweenthese curves. Since the only difference between the two model surveys isthe presence or not of an air-wave component, the ratio plotted in FIG.3 effectively shows the relative strength of the air-wave component inthe detected signal compared to that which passes through thesubterranean strata for the shallow-water model survey.

It is apparent from FIGS. 2 and 3 that at all but the very shortesthorizontal separations the detected electric field is significantlylarger in the shallow-water model. For example, at a horizontalseparation of 2500 m, the amplitude of the detected signal in thedeep-water model survey is around 10⁻¹² Am⁻². In the shallow-water modelsurvey it is higher at around 10^(−11.5) Am⁻². This is due to theadditional contributions of the air-wave component. This level ofincrease shows that the airwave component is over twice as strong as thecomponent which has passed through the subterranean strata, andaccordingly over two-thirds of the detector signal carries almost noinformation about the subterranean strata. At greater horizontalseparations the air-wave component dominates even more. In particular,it becomes especially pronounced beyond around 5000 m. At this pointthere is a break in the rate at which the detected electric fieldamplitude falls with increasing horizontal separation. At a horizontalseparation of around 7000 m, the air-wave component in the shallow waterexample is around twenty times stronger than that which passes throughthe subterranean strata. This clearly imposes high requirements for thesignal-to-noise ratio of data collected over these sorts of horizontalseparations, as is generally the case when a small signal rides on alarge background. It is apparent that the air-wave significantly limitsthe usefulness of these surveying techniques in shallow water.

In addition to the problems associated with the air-wave component,practical EM surveys of the kind shown in FIG. 1 are subject to severalother limitations. These limitations arise because of the need tocarefully consider the orientation of the current flows induced by EMsignals [6]. The response of seawater and subterranean strata (whichwill typically comprise planar horizontal layers) to EM signals isgenerally very different for horizontally and vertically flowing currentcomponents. For horizontally flowing current components, the couplingbetween the layers comprising the subterranean strata is largelyinductive. This means the presence of thin resistive layers (which areindicative of hydrocarbon reservoirs) do not significantly affect the EMfields detected at the surface since the large scale current flowpattern is not affected by the thin layer. On the other hand, forvertical current flow components, the coupling between layers is largelygalvanic (i.e. due to the direct transfer of charge). In these caseseven a thin resistive layer strongly affects the EM fields detected atthe surface since the large scale current flow pattern is interrupted bythe resistive layer. It is known therefore that vertical current loopcomponents of induced current are required to satisfactorily perform anEM survey in the field of oil exploration.

However, sole reliance on the sensitivity of vertical current flowcomponents to the presence of a thin resistive layer can lead toambiguities. The effects on detected EM fields arising from the presencea thin resistive layer can be indistinguishable from the effects arisingfrom other realistic large scale subterranean strata configurations. Inorder to resolve these ambiguities using known survey techniques it isnecessary to determine the response of the subterranean strata to bothhorizontal (i.e. inductively coupled) and vertical (i.e. galvanicallycoupled) induced current flows [6].

Hence it is important when designing a practical EM survey of the kinddescribed above to distinguish between transmitter and receiverconfigurations in which the coupling between layers is largely inductivedue to horizontal currents (in which case the survey has littlesensitivity to the presence of a thin reservoir) and those in which thecoupling between layers is largely galvanic due to vertical currents (inwhich case blocking of the passage of this current flow by a reservoirleads to a survey which is strongly sensitive to the presence andboundary of hydrocarbon within the reservoir).

The HED transmitter 22 shown in FIG. 1 generates both inductive andgalvanic current flow modes with the relative strength of each modedepending on HED transmitter-receiver geometry. At receiver locationswhich are broadside to the HED transmitter axis, the inductive modedominates the response. At receiver locations which are in-line with theHED transmitter axis, the galvanic mode is stronger [6, 8, 9, 10]. Theresponse at receiver locations in both the in-line and broadsideconfigurations is governed by a combination of the inductively andgalvanically coupled modes and these tend to work in opposition.

FIG. 4 shows in plan view an example survey geometry according to theabove described survey method in which sixteen receivers 25 are laid outin a square grid on a section of seafloor 6 above a subterraneanreservoir 56 having a boundary indicated by a heavy line 58. Theorientation of the subterranean reservoir is indicated by the cardinalcompass points (marked N, E, S and W for North, East, South and Westrespectively) indicated in the upper right of the figure. To perform asurvey, a transmitter starts from location ‘A’ and is towed along a pathindicated by the broken line 60 through location ‘B’ until it reacheslocation ‘C’ which marks the end of the survey path. As is evident, thetow path first covers four parallel paths aligned with the North-Southdirection to drive over the four “columns” of the receivers. This partof the survey path moves from location ‘A’ to ‘B’. Starting fromlocation ‘B’, the survey path then covers four paths aligned with theEast-West direction which drive over the four “rows” of receivers. Eachreceiver is thus driven over in two orthogonal directions. The survey iscompleted when the transmitter reaches the location marked ‘C’.

During the towing process, each of the receivers 25 presents severaldifferent orientation geometries with respect to the transmitter. Forexample, when the transmitter is directly above the receiver position D1and on the North-South aligned section of the tow path, the receivers atpositions D5, D6 and D7 are at different ranges in an end-on position,the receivers at positions D2, D3 and D4 are at different horizontalseparations in a broadside position and the receiver at positions D8 andD9 are in-between. However, when the transmitter later passes over thereceiver position D1 when on the East-West aligned section of the towpath, the receivers at positions D5, D6 and D7 are now in a broadsideposition, and the receivers at position D2, D3 and D4 are in an end-onposition. Thus, in the course of a survey, and in conjunction with thepositional information of the transmitter, data from the receivers canbe used to provide details of the signal transmission through thesubterranean strata for a comprehensive range of distances andorientations between transmitter and receiver, each with varyinggalvanic and inductive contributions to the signal propagation. In thisway a simple continuous towing of the transmitter can provide a detailedsurvey which covers the extent of the subterranean reservoir.

While this survey method has been demonstrated to provide good resultsin practice, as noted above some limitations have been identified.

Firstly, since the inductive and galvanic modes cannot be easilyseparated there will generally be a level of cross-talk between them ata receiver. This may lead to ambiguities in the results.

Secondly, in order to obtain survey data from both in-line and broadsidegeometries, the HED transmitter needs to be re-oriented at each HEDtransmitter survey location. This requires the surface vessel to makemultiple passes over broadcast locations and can lead to complex andlong tow patterns.

Thirdly, a HED transmitter based EM survey can only provide the bestdata possible at discrete survey locations. This is because of thegeometric requirements of a HED transmitter survey which dictate that,at any point during the survey, data can only be optimally collectedfrom those receivers to which the HED transmitter is arranged eitherin-line or broadside. At other orientations, horizontal separation ofthe inductively and galvanically coupled signals becomes much moredifficult and data are less reliable. For instance, referring to FIG. 4,when the HED transmitter is at a point on the tow path above thereceiver marked D1 and on the North-South aligned section of the towpath, in-line data can only be collected from the receivers marked D5,D6 and D7, whilst broadside data can only be collected form thereceivers marked D2, D3 and D4. The other receivers provide onlymarginally useful information at this point of the survey. Furthermore,if the HED transmitter is at the location identified by referencenumeral 57 in FIG. 4, which is on a North-South aligned section of thetow path, in-line data can be collected from the receivers marked D3,D8, D9 and D10, but broadside data cannot be collected from any of thereceivers. Since both broadside and in-line data are required foroptimal analysis, the best data possible with the square receiver arrayshown in FIG. 4 can only be collected from points along the tow pathdirectly above the receiver locations.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof analysing results from an electromagnetic survey of an area that isthought or known to contain a subterranean hydrocarbon reservoir,comprising: providing vertical electric dipole (VED) response dataobtained by at least one VED detector detecting a signal from ahorizontal electric dipole (HED) transmitter, providing background dataspecific to the area being surveyed; and comparing the VED response datawith the background data to obtain difference data sensitive to thepresence of a subterranean hydrocarbon reservoir.

In this case, references to vertical and horizontal indicate that asignificant component, preferably a major component, of the respectivesignals should be aligned with the vertical and horizontal axes. It isnot necessary that the signals are perfectly, or even closely, alignedto the vertical and horizontal axes, although fairly close alignment ispreferred to provide a strong signal and reduce the complexity ofanalysis, for example alignment within +/−30° is desirable.

By using VED response data in the analysis, it is possible to analysesurvey results taken in shallower water than has previously beenpossible. This is because the VED response data are not sensitive todetected signal components which have propagated through air and whichtend to dominate survey results obtained with previous methods. Inaddition, because the VED response data are not sensitive to horizontalcurrent flows, there is no mixing between galvanic and inductive modesin the VED response data. This means a complete survey can be carriedout with a single tow. In contrast, in previously proposed EM surveymethods based on use of HED detectors, two sets of survey data had to beobtained with different tows over different paths in order to probe theresponse of an area of interest galvanically and inductively.

The comparison of VED response data with background data makes itpossible to determine whether features of the VED response data areindicative of a hydrocarbon reservoir or arise as a result of some otherlocal background structure configuration. Background data may beobtained by modelling the EM survey performed to obtain the VED responsedata with a model background subterranean strata configuration. Thebackground model strata configuration should preferably be a close matchto the actual background structure in the area being surveyed.

The background data may be obtained in several ways, for example from acontrolled source electromagnetic survey, from a magneto-telluricelectromagnetic survey, from another similar survey taken at a differenttime, or from a rock formation model. If a rock formation model is usedit should preferably include resistivity, and may be derived from acombination of geological data and resistivity data. The geological datacan be from seismological surveying and the resistivity data from welllogging. Other sources of information, such as neutron data or otherporosity estimates from well logs, could also be used.

The difference data may represent the difference between the VEDresponse data and the background data as a function of position withinthe area, and the analysis may include identifying a location of aboundary of a subterranean hydrocarbon reservoir.

According to a second aspect of the invention there is provided acomputer program product bearing machine readable instructions forimplementing a method of analysing results from an electromagneticsurvey according to the first aspect of the invention.

According to a third aspect of the invention there is provided acomputer apparatus loaded with machine readable instructions forimplementing the method of analysing results from an electromagneticsurvey according to the first aspect of the invention.

According to a fourth aspect of the invention there is provided anelectromagnetic survey method applied to an area that is thought orknown to contain a subterranean hydrocarbon reservoir, comprising:deploying a horizontal electric dipole (HED) transmitter and at leastone receiver, the receiver including a vertical electric dipole (VED)detector; and collecting from the at least one VED detector VED responsedata induced by the HED transmitter.

An electromagnetic survey method of this type is well suited toproviding VED response data for use in analysing results from anelectromagnetic survey according to the first aspect of the invention.

The VED detector may consist of a single vertically aligned antenna. Inanother embodiment, three antennae can be used, one vertical twohorizontal, each ideally being orthogonal to each other. However, aslong as the two horizontal antennae are generally in crossed alignment,and extending in different horizontal directions, this will besufficient.

According to a fifth aspect of the invention there is provided a methodof planning an electromagnetic survey of an area that is thought orknown to contain a subterranean hydrocarbon reservoir, comprising:creating a model of the area to be surveyed including a rock formationcontaining a postulated hydrocarbon reservoir, and a body of water abovethe rock formation; setting values for water depth, depth of thepostulated hydrocarbon reservoir, and resistivity structure of the rockformation; and performing a simulation of an electromagnetic survey inthe model of the survey area by calculating vertical electric dipole(VED) response data obtained by at least one simulated VED detectordetecting a signal from a simulated horizontal electric dipole (HED)transmitter; and adjusting the model to remove the postulatedhydrocarbon reservoir and repeating the simulation to obtain backgrounddata for comparison with the VED response data.

Repeated simulations for a number of transmitter-receiver horizontalseparations and frequencies of signal can be performed in order to allowoptimum surveying conditions in terms of transmitter-to-detectordistance and frequency of EM signal for probing the hydrocarbonreservoir to be selected when performing an electromagnetic survey. Theeffects and usefulness of differing detector array configurations andtransmitter tow paths can also be modelled.

According to a sixth aspect of the invention there is provided acomputer program product bearing machine readable instructions forimplementing the method of planning an electromagnetic survey accordingto the fifth aspect of the invention.

According to a seventh aspect of the invention there is provided acomputer apparatus loaded with machine readable instructions forimplementing the method of planning an electromagnetic survey accordingto the fifth aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings, in which:

FIG. 1 shows in schematic vertical section a surface vessel undertakingan EM survey in deep water according to a previously proposed method;

FIG. 2 is a graph plotting detector signals calculated from two modelsurveys made according to the previously proposed method, one performedin deep water (dotted line) and one performed in shallow water (solidline);

FIG. 3 is a graph plotting the ratio of the two curves shown in FIG. 2;

FIG. 4 is a schematic plan view showing an example survey geometryaccording to the previously proposed method in which sixteen detectorsare laid out on a section of seafloor above a subterranean reservoir;

FIG. 5 shows in schematic vertical section a surface vessel undertakingan EM survey in shallow water according to an embodiment of theinvention;

FIG. 6 is a graph plotting detector signals calculated from two modelsurveys made according to the embodiment of the invention, one performedin deep water (dotted line) and one performed in shallow water (solidline);

FIG. 7 is a graph plotting the ratio of the two curves shown in FIG. 6;and

FIG. 8 is a schematic plan view showing an example survey geometry forsurveying according to an embodiment of the invention in which sixteendetectors are laid out on a section of seafloor above a subterraneanreservoir.

DETAILED DESCRIPTION

FIG. 5 of the accompanying drawings schematically shows a surface vessel14 undertaking electromagnetic (EM) surveying of a subterranean strataconfiguration (or rock formation) according to an embodiment of theinvention. Features shown in FIG. 5 which are functionally similar tofeatures shown in FIG. 1 are given the same reference numeral. Thesubterranean strata configuration includes an overburden layer 8, anunderburden layer 9 and a hydrocarbon reservoir 12. The surface vessel14 floats on the surface 2 of the sea 4. A submersible vehicle 19 isattached to the surface vessel 14 by an umbilical cable 16 whichprovides an electrical, optical and mechanical connection between thesubmersible vehicle 19 and the surface vessel 14. The submersiblevehicle 19 is towed by the surface vessel 14 such that it remainsconsistently close to the seafloor 6. This is facilitated by anecho-location package 20 which relays information about the height ofthe towed vehicle 19 above the seafloor 6 to the surface vessel 14. Thetowed vehicle 19 receives electrical power from the ship's on-boardpower supply via the umbilical cable 16.

A waveform generator in the form of a cycloconverter unit 30 generates aselected waveform for an alternating current (AC) drive current. The ACdrive current is supplied to a horizontal electric dipole (HED)transmitter 22 which is towed by the submersible vehicle 19. Whilst inthis example a cycloconverter is used, any waveform generator capable ofgenerating a suitable output signal may be employed. Furthermore,although in this example the waveform generator is aboard the ship, inother examples the waveform generator may be on the towed vehicle. Inthese cases it may be supplied with electrical power from the surfacevessel's power supply. The supply of the AC drive current to the HEDtransmitter causes it to broadcast a HED EM signal into the sea 4.

One or more remote receivers 35 are located on the seafloor 6. Thereceivers 35 comprise an instrument package 36, an antenna 34, afloatation device 38 and a ballast weight (not shown). The antenna 34comprises a vertical electric dipole (VED) detector and two mutuallyorthogonal horizontal electric dipole (HED) detectors. A standard marinecompass may be included in the instrument package to record theorientation of the HED detectors. In other examples, only the VEDdetector is included (i.e. the HED detector pair is omitted). The VEDdetector is sensitive to vertical electric components of the EM fieldsinduced by the HED transmitter in the vicinity of the receiver 35 andmay be a simple VED antenna The VED dipole produces detector signalsforming VED response data. The instrument package 36 records the VEDresponse data for later analysis. At the end of a survey, a remotelyoperable release system allows the instrument package 36 to be detachedfrom the ballast weight so that the floatation device 38 can carry theinstrument package to the surface for recovery and retrieval of the VEDresponse data for analysis.

The vertical components of the EM fields induced by the HED transmitterat the detector are significantly weaker than the horizontal components.This is perhaps one reason why recent development of electromagneticsurveying for detecting hydrocarbon reservoirs has been focussed arounduse of HED receivers. However, in using a VED detector to obtain VEDresponse data as taught by the invention, surveys which are far lesssensitive to the air-wave component can be performed. This is becausethe air-wave component is strongly biased towards inducing horizontalcurrents at the detector. The component of the air-wave which wouldinduce vertical currents at the detector is highly inefficiently coupledacross the horizontal interface between the air and the seawater.Because the air-wave component is unable to dominate the VED responsedata, the proposed survey technique is able to operate in water which ismuch shallower water than previously described methods.

FIG. 6 is a graph schematically showing results of one-dimensionalmodelling of two example EM surveys of the kind shown in FIG. 5. FIG. 6is to be compared with FIG. 2 showing results of modelling of previouslyknown EM surveys. Again, one example in FIG. 6 corresponds to a surveyperformed in deep water (dotted line) and the other to a surveyperformed in shallow water (solid line). Unlike FIG. 2, it is theamplitude of the vertical electric field component induced at thereceiver in response to the HED transmitter calculated per unittransmitter dipole which is plotted as a function oftransmitter-receiver horizontal separation R in FIG. 6. For both thedeep-water and the shallow-water model surveys the subterranean strataconfiguration is again a semi-infinite homogenous half space ofresistivity 1 Ωm. As before, in the deep-water model survey the seawateris of infinite extent, and in the shallow-water model survey it is ofdepth 500 meters. In both cases the seawater has resistivity 0.3 Ωm. Thetransmitter and receiver are again separated along a line which runsthrough the axis of the HED transmitter and the HED transmitter isdriven by a quasi-square wave AC current at a frequency of 0.25 Hz.

It is apparent from FIG. 6 that in using the survey technique of theinvention, the effect of the air-wave component on the amplitude of EMfields induced by the HED transmitter at the receiver is minimal. Inboth the deep-water model survey, where there is no air-wave component,and the shallow water model survey, where there is an air-wavecomponent, the calculated electric field amplitudes are very similar.FIG. 7 is a plot showing the ratio, p, of the two curves shown in FIG.6. FIG. 7 is to be compared with FIG. 3 and is plotted on the same scaleto allow direct comparison. The similarity of the VED response datacalculated for both the deep-water and shallow-water model surveys isapparent from the curve in FIG. 7 being close to unity for allhorizontal separations. A comparison of FIG. 7 with FIG. 3 provides aclear demonstration of the improved EM survey performance obtainable inshallow water using the new survey technique.

In the example shown in FIG. 5, the VED detector is rigidly attached tothe remainder of the receiver structure. This is to prevent the VEDdetector moving in response to undersea water currents. Motion of theVED detector would otherwise introduce noise into the VED response data,for example from currents induced by its motion in the Earth's magneticfield. Because the VED detector is rigidly attached to the remainder ofthe receiver, it may not always be correctly vertically aligned. Thiswould be the case, for example, if the receiver was deployed on asloping surface. To account for this, and to ensure the verticalcomponent of the EM fields induced by the HED transmitter at thedetector are properly measured, the instrument package preferablyincludes a clinometer so that the detector signals can be properlyresolved into their vertical and horizontal components. In anotherexample, a naturally buoyant VED detector may be employed. This could,for example, be provided by a dipole detector pivotably attached to theremainder of the receiver at one end and having a buoyancy aid attachedto the other end. A clinometer may again be included to allow detectorsignals to be properly resolved. Such an arrangement would ensure thatthe dipole detector is vertically aligned irrespective of theorientation of the remainder of the receiver. However, as noted above,this would make the VED detector sensitive to motionally induced noise.This could be reduced, for example, by employing a clamping mechanism torigidly hold the pivotably attached dipole detector once it has settledin a vertical alignment following initial deployment.

The characteristics of the modelled VED response data shown in FIG. 6are specific to the subterranean strata configuration employed in themodel surveys. In both cases, this is a semi-infinite homogeneousstructure with a resistivity of 1 Ωm. If the subterranean strataconfiguration were different, calculated curves corresponding to thoseof FIG. 6 would also be different. For example, if there were ahydrocarbon reservoir embedded within the subterranean strataconfiguration, such as the one schematically shown in FIG. 5, thedetected electric field amplitudes would be higher. This is due to thelower attenuation experienced by the components of the EM signal fromthe HED transmitter which travel through the higher resistivityhydrocarbon reservoir. The magnitude of the increase in signal due to ahydrocarbon reservoir is likely to be different at differenttransmitter-receiver horizontal separations R. This is because thesignal propagation between the HED transmitter and the VED detector isweighted by depth as a function of horizontal separation between the HEDtransmitter and the VED detector. For example, if the HED transmitterand the VED detector are close compared to the depth of a buriedhydrocarbon reservoir, effects of signals propagating through thehydrocarbon reservoir will be small since the detector signals will bedominated by signals which have propagated through the overburdenlayers. However, at larger horizontal separations, the effects of thehydrocarbon reservoir will become more pronounced. It is these types ofeffect which make the technique described above sensitive to thepresence of hydrocarbon reservoirs.

Because in practice, subterranean strata configurations are not assimple as those used in the model surveys described above, it is oftendifficult to identify directly from curves of the type shown in FIG. 6which have been obtained from real surveys whether the curves containfeatures indicative of a buried hydrocarbon reservoir or merely featuresrelating to local larger scale background structures. In particular thekind of VED response data seen with a thin resistive hydrocarbonreservoir embedded in a uniform resistivity background is similar tothat seen in a subterranean strata configuration comprising layers ofincreasing resistivity with depth. This kind of increasing-resistivitystructure is a feature of some submarine sedimentary basins and canarise due to the progressive expulsion of conductive pore fluids withincreasing depths by a rising overburden pressure. Accordingly knowledgeof the large scale background structure of the subterranean strata inthe area to be surveyed is often needed in order to determine reliablywhether features in VED response data are caused by a buried hydrocarbonlayer or whether they are caused by large scale background structures.

A survey aimed at establishing whether a subterranean strataconfiguration contains a thin resistive hydrocarbon reservoir willnormally involve obtaining VED response data, of the type plotted inFIG. 6, using the technique shown in FIG. 5. These VED response data aresensitive to the presence of subterranean hydrocarbon reservoirs.However, in addition, to determine reliably whether features of the VEDresponse data are indicative of a hydrocarbon reservoir or of the localbackground structure, it is necessary to determine how the VED responsedata for a given subterranean strata configuration would appear if therewere no hydrocarbon reservoir.

This analysis step, generally referred to as normalisation, is usuallydone with the aid of background data. Background data are specific tothe area being surveyed and can be obtained in a variety of ways. Oneway is to model the EM survey performed to obtain the VED response datawith a model background subterranean strata configuration. Thebackground model should be as close a match as possible to the actualbackground structure in the area being surveyed. A comparison of the VEDresponse data with the background data provides difference datasensitive to the likely presence, extent and location of a subterraneanhydrocarbon reservoir embedded within the background subterranean strataconfiguration. For example, if the VED response data closely match thebackground data, there is unlikely to be a buried hydrocarbon layer. If,on the other hand, there are differences, i.e. anomalies, in the VEDresponse data compared to the background data, for example, an increasedreceiver signal amplitude, this could be analysed quantitativelyassessed in terms of being indicative of a buried hydrocarbon reservoir.The variation in anomalies at different horizontal separations providesinformation on the depth and extent of a hydrocarbon reservoir. Forexample, if differences between the VED response data and the backgrounddata are only apparent at large transmitter-receiver horizontalseparations, this is likely to indicate that the hydrocarbon reservoiris relatively deeply buried. Similarly, a discontinuity in VED responsedata as a function of horizontal separation is likely to indicate aboundary or edge of a hydrocarbon reservoir at the location of thediscontinuity.

Suitable background models to use in generating background data can beobtained in several ways.

One way of obtaining the information required to construct a suitablebackground model is with conventional MT electromagnetic surveyingtechniques. As noted above, these techniques are capable of providinginformation on large scale background resistivity structures, eventhough they unable to detect hydrocarbon reservoirs directly.

Another way of obtaining the information required to construct asuitable background model is by using conventional controlled sourceelectromagnetic surveying techniques. Whilst the use of such techniquesto determine a background model is prone to the same difficultiesassociated with the air-wave component described above, the impact ofthe air-wave component can be reduced by employing relatively lowfrequency EM signals. Low frequency signals suffer less attenuation asthey pass through the subterranean strata and so the air-wave componentis not so dominant in the EM fields induced at a receiver by a HEDtransmitter driven by a low frequency AC current. While these lowfrequency signals are unlikely to provide sufficient spatial resolutionto properly resolve subterranean hydrocarbon reservoirs, they arenonetheless capable of providing information on large scale backgroundresistivity structures needed to generate a background model.

In other cases, an area to be surveyed will already be very wellcharacterized by previous surveying. For example, in a producingoilfield or oil province there is likely to be a wealth of existingseismic and well-log data. In these cases, background models can becalculated from a rock formation model. The rock formation model can becreated from the seismic data and then resistivities assigned to thevarious components in the rock structure using the resistivitiesobtained from well-log data. (If directly applicable well-log data arenot available, it may be possible to estimate resistivity values bycomparison with resistivity data from nearby wells in similar geologicalstructures.) This technique for obtaining the information required toconstruct a suitable background model will be especially suited toapplications in existing oilfields, such as monitoring long termdepletion of reserves.

When monitoring depletion, it may be sufficient to directly compare VEDresponse data taken at different times, e.g. several weeks or monthsapart, without use of a rock formation model. In other words, thebackground data used is data from a previous similar survey. Differencesin VED response data taken at different times are indicative of changesin the hydrocarbon reservoir which have occurred between the times atwhich the data were taken. Because of this, this kind of comparisonprovides a useful monitoring tool. The VED response data taken at theearlier time thus effectively acts as background data for comparing withthe VED response data taken at the later time.

Having described some of the types of survey results required, and thesteps involved in analysing them, practical methods of performing an EMsurvey of an area of interest are now considered.

In order to perform a thorough survey over a large area, and to providecurves similar to those shown in FIG. 6, controlled sourceelectromagnetic measurements will typically be made with many HEDtransmitter and receiver locations. Whilst these could be made inparallel using a plurality of HED transmitters operating at differentfrequencies and a plurality of receivers, or in series using a singleHED transmitter and a single receiver which are repositioned betweenmeasurements, it will generally be more efficient to employ a pluralityof receivers in fixed positions and a single HED transmitter. The HEDtransmitter may be mobile, as in FIG. 5, e.g. for surveying new areas,or may be static, e.g. for monitoring depletion in an existing oilfield.

The HED transmitter 22 shown in FIG. 5 can require significant power todrive it, of the order tens of kilowatts, or greater for signalsdetectable at horizontal separations of several kilometers. Theumbilical cable 16 connecting the HED transmitter 22 to the surveyvessel 14 supplies this power and makes it relatively straightforward tomake the EM transmitter mobile. It can then be towed in an appropriatesurvey pattern by the surface vessel 14. Since in many situationssurveys of this kind are liable to take place over areas of the seafloor6 where sensitive engineering installations exist or are planned, thereare significant advantages to using a transmitter which does notphysically come into contact with the seafloor 6. Provided that thehorizontal separation between the HED transmitter 22 and the seafloor 6is small compared to a skin depth of the investigating field inseawater, the survey can still be performed.

As described above, an HED transmitter of the kind shown in FIG. 5induces a combination of vertically and horizontally oriented currentloops at the receivers depending on the relative transmitter-receiverorientations. It is this mode mixing which required previously proposedsurveys to follow complicated tow paths of the kind shown in FIG. 4.However, since a VED detector is not sensitive to horizontal currentflows, only galvanically coupled modes contribute to the VED responsedata. Because of this lack of mixing of the galvanically and inductivelycoupled modes in the VED response data, the complicated surveygeometries required with previously proposed methods are not requiredwhen surveying according to the method shown in FIG. 5. There are,however, some geometrical limitations associated with the new surveyingtechnique. This is because with a uniformly layered subterranean strataconfiguration, there is no vertical component in the EM fields inducedby a HED transmitter at a receiver located exactly broadside to the HEDtransmitter. Accordingly, no signal will be received in thisconfiguration. The angular range over which VED response data canusefully be collected will depend on the sensitivity of the VEDdetector, the strength of the HED transmitter and thetransmitter-receiver horizontal separation. However, so long as there isa detectable vertical component to the EM fields induced by the HEDtransmitter at the receiver, VED response data obtained for anytransmitter-receiver orientation can be used without the problemsassociated with mixed galvanically and inductively coupled modes.

FIG. 8 is a schematic plan view showing an example layout of sixteenreceivers 35 distributed across a section of seafloor 6 abovesubterranean reservoir 56 during a survey of the kind shown in FIG. 5.The reservoir 56 has a linear extent on the order of several km and itsboundary is indicated by a heavy line 58. The reservoir is similar tothe reservoir shown in FIG. 4. The orientation of the subterraneanreservoir is indicated by the cardinal compass points (marked N, E, Sand W for North, East, South and West respectively) indicated in theupper right of the figure. In this example, the receivers 35 areuniformly distributed in a square-grid pattern so as to approximatelycover the reservoir 56. In performing the survey, a HED transmitter (notshown) starts from the location marked ‘A’ in FIG. 8 and is towed,whilst broadcasting continuously as described above, along a pathindicated by the broken line 60. The survey is completed when thetransmitter reaches the location marked ‘B’. VED response data arecontinuously collected by the receivers 35 throughout the towing processand the position of the HED transmitter relative to the network ofreceivers is also logged.

During the towing process, each of the receivers 35 presents severaldifferent transmitter-receiver horizontal separations and orientationsrelative to the HED transmitter. Accordingly, by following the tow pathmarked, VED response data are collected for many differenttransmitter-receiver horizontal separations along many differentdirections. These VED response data can be inverted to provide athorough map of the subterranean strata configuration using appropriategeophysical inversion techniques. In this way a simple continuous towingof the transmitter can provide a detailed survey which covers the extentof the subterranean reservoir 56.

As described above, FIG. 4 shows for comparison an example tow pathwhich might be followed in performing a previously known survey of thekind shown in FIG. 1. The tow path shown in FIG. 4 is approximatelytwice as long as that shown in FIG. 8. Furthermore, when performing anEM survey according FIG. 8, VED response data can be collected when theHED transmitter is at all points along a tow path as long as the towpath direction is suitable.

Collecting VED response data thus allows a greater amount of useful datato be collected from a shorter tow path than the previous proposedend-on/broadside method based on collection of HED response data.

Although the above example is based on a square receiver grid, it willbe understood that a wide variety of receiver placements may be used.For example other high symmetry regular grids, such as triangular orrectangular, may be used. In addition irregular grids may be used thathave no high level of symmetry.

In the case of the receivers, there are further advantages in usingstatic devices. Firstly, receivers suitable for EM survey techniques ofthe kind shown in FIG. 5 can be internally powered and are relativelycheap compared to HED transmitters. Because of this a plurality ofreceivers can easily by arrayed around an area of interest so as toprovide multiple transmitter-receiver horizontal separations andpositions for a single EM transmitter position, as shown in FIG. 8.Secondly, the task of the receivers is to measure electric fields at theseafloor 6. In typical applications, the signal-to-noise ratio of thedetector signals is important to the success and resolution of a survey,and should be maximised. Moving a receiver inevitably generates noise.Any motion of the receiver through the conducting seawater 4 in thepresence of the Earth's magnetic field generates an electromotive forceacross the VED detector. Receiver movements will therefore map intospurious detector signals in the VED response data.

For these reasons it is desirable to carry out a controlled sourceelectromagnetic survey to investigate or detect the presence of buriedhydrocarbons using a combination of a mobile HED transmitter operatedjust above the seafloor and an array of detectors placed statically onthe seafloor as indicated in FIG. 8. At the end of a survey, theinstrument packages associated with the receivers are recovered using anacoustically actuated release mechanism to separate it from the ballastweight, allowing them to return to the sea surface for recovery,following standard oceanographic and marine geophysical practice.

Transmitter and receiver positions can be determined using standard longand/or short and/or ultra-short baseline acoustic navigation systems andprocessed to determine the horizontal separation between the transmitterand receiver.

It can be seen from FIG. 8 that with the presently proposed surveytechnique, subterranean strata can be comprehensively surveyed withrelatively few receivers and EM transmitter tows. This reduces the timetaken, and hence cost, of an exploratory EM survey.

It will be understood that whilst the above description describes atowed HED transmitter, the method would also be applicable in a fixedinstallation. For example, the method could be used to monitor changesto a hydrocarbon reservoir from which hydrocarbon is being drawn. Insuch cases it will be appropriate to employ one (or more) HEDtransmitter(s) in fixed positions relative to a receiver array ratherthan undertake frequent towed surveys. The HED transmitter(s) could beanchored to the seafloor or suspended from an oil-rig platform, forexample. In other examples, the HED transmitter(s) could be placed in ahorizontal well or borehole, e.g. a geotechnical borehole. In the caseof a producing oil field, the subterranean structures are likely to bewell known already from prior geophysical surveys and drilling results.Indeed, prior geophysical and geological information from the oil fieldcan be used to construct a background model as described above.

Finally, it will be understood that the invention is equally applicableto surveying of freshwater, for example large lakes or estuaries, sothat references to seafloor, seawater etc. should not be regarded aslimiting and should be interpreted as covering lakebed, riverbed etc.Indeed the applicability of the invention to shallow water makes itideal for surveying shallow lakes.

REFERENCES

-   [1] Sinha, M. C., Patel, P. D., Unsworth, M. J., Owen, T. R. E. &    MacCormack, M. R. G. An active transmitter electromagnetic sounding    system for marine use. Mar. Geophys. Res., 12, 1990, 59-68.-   [2] Evans, R. L., Sinha, M. C., Constable, S. C. & Unsworth, M. J.    On the electrical nature of the axial melt zone at 13°N on the East    Pacific Rise. J. Geophys. Res., 99, 1994, 577-588.-   [3] Edwards, R. N., Law, K. L., Wolfgram, P. A., Nobes, D. C.,    Bone, M. N., Trigg, D. F. & DeLaurier, J. M., First result of the    MOSES experiment: Sea sediment conductivity and thickness    determination, Bute Inlet, Columbia, by magnetometric offshore    electrical sounding, Geophyics, 50, 1985, 153-16.-   [4] WO 00/13046 A1-   [5] WO 01/57555 A1-   [6] Eidesmo, T., Ellingsrud, S., MacGregor, L. M., Constable, S.,    Sinha, M. C., Johansen, S, Kong, F-N & Westerdahl, H., Sea Bed    Logging (SBL), a new method for remote and direct identification of    hydrocarbon filled layers in deepwater areas, First Break, 20, 2002,    144-152.-   [7] Ellingsrud, S., Eidesmo, T., Johansen, S., Sinha, M. C,    MacGregor, L. M. & Constable, S., Remote sensing of hydrocarbon    layers by seabed logging (SBL): Results from a cruise offshore    Angola, The Leading Edge, October 2002. 972-982.-   [8] MacGregor, L. M. & Sinha, M. C. Use of marine controlled source    electromagnetic sounding for sub-basalt exploration. Geophysical    Prospecting, 48, 2000, 1091-1106.-   [9] WO 02/14906 A1-   [10] MacGregor, L. M., Constable, S. C. & Sinha, M. C. The RAMESSES    experiment III: Controlled source electromagnetic sounding of the    Reykjanes Ridge at 57° 45′ N. Geophysical Journal International,    135, 1998, 773-789.

1. A method of analysing results from an electromagnetic survey of anarea that is thought or known to contain a subterranean hydrocarbonreservoir, comprising: providing vertical electric dipole (VED) responsedata obtained by at least one VED detector detecting a signal from ahorizontal electric dipole (HED) transmitter; providing background dataspecific to the area being surveyed; and comparing the VED response datawith the background data to obtain difference data sensitive to thepresence of a subterranean hydrocarbon reservoir.
 2. A method ofanalysing results from an electromagnetic survey according to claim 1,wherein the background data are obtained from a controlled sourceelectromagnetic survey.
 3. A method of analysing results from anelectromagnetic survey according to claim 1, wherein the background dataare obtained from a magneto-telluric electromagnetic survey.
 4. A methodof analysing results from an electromagnetic survey according to claim1, wherein the background data are further VED response data obtainedfrom another electromagnetic survey of the area performed at a differenttime.
 5. A method of analysing results from an electromagnetic surveyaccording to claim 1, wherein the background data are calculated from arock formation model.
 6. A method of analysing results from anelectromagnetic survey according to claim 5, wherein the rock formationmodel is derived from a combination of geological data and resistivitydata.
 7. A method of analysing results from an electromagnetic surveyaccording to claim 6, wherein the geological data are from seismologicalsurveying.
 8. A method of analysing results from an electromagneticsurvey according to claim 6, wherein the resistivity data are from welllogging.
 9. A method of analysing results from an electromagnetic surveyaccording to claim 1, wherein difference data are obtained as a functionof position within the area and are sensitive to boundaries of thesubterranean hydrocarbon reservoir.
 10. A computer program productcomprising a machine readable medium bearing machine-executableinstructions for implementing a method of analysing results from anelectromagnetic survey according to claim
 1. 11. A computer apparatusloaded with machine executable instructions for implementing the methodof analysing results from an electromagnetic survey according toclaim
 1. 12. An electromagnetic survey method applied to an area that isthought or known to contain a subterranean hydrocarbon reservoir, thearea comprising subterranean strata beneath a seafloor, the methodcomprising: deploying at or above the seafloor a horizontal electricdipole (HED) transmitter and at least one receiver, the receiverincluding a vertical electric dipole (VED) detector; and collecting fromthe at least one VED detector VED response data induced by the HEDtransmitter.
 13. An electromagnetic survey method according to claim 12,wherein the receiver comprises a single vertically aligned antenna. 14.An electromagnetic survey method according to claim 12, wherein thereceiver comprises a vertically aligned antenna and two horizontallyaligned antennae extending in different horizontal directions.
 15. Anelectromagnetic survey method according to claim 12, further comprising:providing background data specific to the area being surveyed; andcomparing the VED response data with the background data to obtaindifference data sensitive to the presence of a subterranean hydrocarbonreservoir.
 16. A method for obtaining hydrocarbon from an area thatcontains a subterranean hydrocarbon reservoir, comprising: providingvertical electric dipole (VED) response data obtained by at least oneVED detector detecting a signal from a horizontal electric dipole (HED)transmitter during an electromagnetic survey of the area; providingbackground data specific to the area being surveyed; comparing the VEDresponse data with the background data to obtain difference datasensitive to the presence of the subterranean hydrocarbon reservoir;identifying the subterranean hydrocarbon reservoir using the differencedata; and penetrating the subterranean hydrocarbon reservoir with ahydro-carbon producing well.
 17. A volume of hydrocarbon obtained froman area that contains a subterranean hydrocarbon reservoir, thehydrocarbon obtained by: providing vertical electric dipole (VED)response data obtained by at least one VED detector detecting a signalfrom a horizontal electric dipole (HED) transmitter during anelectromagnetic survey of the area; providing background data specificto the area being surveyed; comparing the VED response data with thebackground data to obtain difference data sensitive to the presence ofthe subterranean hydrocarbon reservoir; identifying the subterraneanhydrocarbon reservoir using the difference data; penetrating thesubterranean hydrocarbon reservoir with a hydrocarbon-producing well;and extracting the volume of hydrocarbon from the reservoir using thewell.
 18. A results data set representing an area that is thought or isknown to contain a subterranean hydrocarbon reservoir, the results dataset obtained by: deploying at or above the seafloor a horizontalelectric dipole (HED) transmitter and at least one receiver, thereceiver including a vertical electric dipole (VED) detector; collectingfrom the at least one VED detector VED response data induced by the HEDtransmitter; and generating the results data set based on the VEDresponse data.
 19. A computer readable storage medium having a resultsdata set according to claim 18 recorded thereon.
 20. A method forobtaining hydrocarbon from an area that contains a subterraneanhydrocarbon reservoir, comprising: extracting hydrocarbon from thesubterranean hydrocarbon reservoir, the subterranean hydrocarbonreservoir having been determined to contain hydrocarbon by means of anelectromagnetic survey method comprising the steps of: providingvertical electric dipole (VED) response data obtained by at least oneVED detector detecting a signal from a horizontal electric dipole (HED)transmitter during an electromagnetic survey of the area; providingbackground data specific to the area being surveyed; comparing the VEDresponse data with the background data to obtain difference datasensitive to the presence of the subterranean hydrocarbon reservoir; andidentifying the subterranean hydrocarbon reservoir using the differencedata.
 21. A method according to claim 20, wherein the extracting stepincludes penetrating the subterranean hydrocarbon reservoir with ahydrocarbon-producing well.